The present invention relates to a method for threading a material web through a processing plant.
The method is specifically adapted for threading a material web through a processing plant, in which the material web, in alternating directions, passes through two or more decks, and in particular when the processing plant is adapted to tension the material web by regulating the speed of one or more conveying cylinders so that somehow established tractive force is transferred to the material web.
Material webs, such as pulp webs or paper webs, are in technical contexts processed with widths of several meters and at considerable web speeds. Moreover, transfer often occurs between two or more processing steps, in which an accurate control is necessary to prevent operational disorder.
Especially when starting operation, the transfer between processing steps is a most critical point. When transferring a material web between two processing steps, one therefore usually begins with a narrow strip at one edge of the material web, a so-called leader. The leader is pulled through the processing step and then the width of the material to be processed is successively increased until finally the entire width is reached. The part separated during the threading is rejected or recycled for reworking.
If the processing plant comprises more than two steps, the threading process must be repeated in each transition. This means that the reliability and speed of a threading method is most important to efficiency and economic yield. Each failure costs a lot of money.
Originally the width of the leader is purposely very small relative to the full width of the material web. As the successive increase of the width proceeds, it may during the threading, in one and the same processing step, be a web of material with a width from e.g. 0.1 m to 6 m. This means that the force by which the web is pulled through the processing step must be controlled most accurately. The length of the web in a processing step can, e.g. in paper and pulp dryers, be several hundreds of meters. However, the critical point is where the web enters a drier since the low dry solids content then gives the lowest strength.
One example of a close prior-art method is described in U.S. pat. No. 5,158,648. This publication describes in detail the established technique using an edge strip in connection with threading and the drawbacks involved therein. As an improvement it is suggested that the web be broadened symmetrically starting from a central point. To this end, use is made of two knives which are freely movable over the width of the web. This is said to prevent lateral movement and flapping of the web.
An operator monitors the process and controls the retrieval of slack and regulates the tension of the web.
An object of the invention is to provide a quick and reliable method for threading a material web.
In particular the invention aims at providing a quick and reliable method for threading in transferring a web of pulp from the wet end to a dryer when manufac- turing papermaking pulp and in transferring a paper web from the wet end to a dryer when making paper.
The present invention relates to a method for threading a material web through a processing plant. The material web is divided, by a longitudinal cut, into a first narrow part and a second broad part, the first part being passed through the processing plant while the second part is separated. The width of the first part is successively increased so that a growing share of the web-shaped material is passed through the processing plant. Finally, the entire width of the material web is passed through the processing plant. The material web is pulled through the processing plant by a controllable force (tension).
In the method according to the invention, the magni- tude of the controllable force is automatically adjusted to the width of the first part of the material web, preferably so that the magnitude of the force is selected proportional to the width of the first part.
When threading a material web through a processing plant, the web is divided, by a longitudinal cut, into a first narrow part and a second broad part, the first part being passed through the processing plant while the second part is separated. The width of the first part is successively increased so that a growing share of the material web is passed through the processing plant. This is a critical phase in the production of, for example, paper. The risk of repeated breaks of the web with the ensuing long down- times is obvious. The conventional method of controlling the force by which the leader is pulled through the dryer is that an operator first performs the retrieval of the slack forming on the occasion of starting and subsequent- ly manually increases the tractive force in the web, the so-called web tension, so that the web is kept suffi- ciently tensioned but is not subjected to such stress as results in web break.
According to the present invention it is suggested that the magnitude of the controllable force that pulls the web through the dryer be automatically adjusted to the width of the first part, the leader, of the material web. This should in the first place occur in such manner that the magnitude of the force is selected proportional to the width of the first part.
The preferred principle is that the magnitude of the force is adjusted proportional to the width of the web when entering the dryer. This can be carried out, for example, by synchronous control of the position of the knife dividing the web and the tractive force giving the tension to the web. A further possibility is that the width of the web is measured in the vicinity of the web entering the dryer and that this measured valve is allow- ed to control the tractive force.
After an initial interval with an essentially con- stant width the width of that part which is passed through the processing plant is successively increased. That may occur continuously, but occurs suitably through at least two monotonously growing phases with an interme- diate interval with an essentially constant width, pre- ferably through three or more monotonously growing phases with intermediate intervals with essentially constant widths.
If the material web, in alternating directions, passes through two or more decks, the length of the intermediate interval or intervals should exceed the length of the web located in an individual deck, Suitably the length of the intermediate interval or intervals is smaller than twice the length of the web located in an individual deck. The length of at least one monotonously growing phase should be smaller than the length of the web located in an individual deck. In a preferred embodiment, the length of each of two or more monotonously growing phases, preferably the first phases, is smaller than the length of the web located in an individual deck.
The width of the first part during the initial interval should be 50-200 mm, preferably about 100 mm.
The width of the first part during one or more mono- tonously growing phases should be increased by a factor 2 to 5.